Glass substrate assembly, semiconductor device and method of heat-treating glass substrate

ABSTRACT

Improved method of heat-treating a glass substrate, especially where the substrate is thermally treated (such as formation of films, growth of films, and oxidation) around or above its strain point. If devices generating heat are formed on the substrate, it dissipates the heat well. An aluminum nitride film is formed on at least one surface of the substrate. This aluminum nitride film acts as a heat sink and prevents local concentration of heat produced by the devices such as TFTs formed on the glass substrate surface.

This application is a Divisional application of Ser. No. 08/462,773filed Jun. 5, 1995 (now U.S. Pat. No. 5,929,487); which itself is aDivisional of application of Ser. No. 08/311,275, filed Sep. 23, 1994(now U.S. Pat. No. 5,674,304).

FIELD OF THE INVENTION

The present invention relates to a glass substrate on which asemiconductor device is formed and, more particularly, to a bottom layerformed on the surface of a glass substrate and a method of fabricatingthe bottom layer. Furthermore, the invention relates to a method ofheat-treating a glass substrate.

BACKGROUND OF THE INVENTION

A thin-film transistor (TFT) is known as a thin-film semiconductordevice fabricated on a glass substrate. TFTs formed on such a glasssubstrate are disposed in a pixel driver portion and also in aperipheral circuit for a liquid crystal display and are used to displayimages with high information content. Furthermore, these TFTs areemployed in image sensors and in other integrated circuits.

Where a glass substrate is used, the following advantages can bederived:

(1) Since it is optically transparent to visible light, the glasssubstrate can be easily utilized in a device such as a liquid crystaldisplay through which light is transmitted.

(2) It is inexpensive. However, the upper limit of the thermal treatmenttemperature is restricted by the heatproofness, i.e., the maximum usabletemperature, of the glass substrate.

Corning 7059 glass is generally used as a glass substrate taking accountof the problem of impurity release from the glass substrate, priceproblem, and other problems. The transition point of this glass is 628°C. and the strain point is 593° C. Other known practical industrialglass materials having strain points of 550-650° C. are listed in Table1 below.

TABLE 1 7059D(CGW) 7059F(CGW) 1733(CGW) LE30(HOYA) TRC5(OHARA)E-8(OHARA) N-0(NEG) OA2(NEG) strain 593 593 640 625 643 625 point (° C.)thermal 50.1 50.1 36.5 38.0 52.0 37.0 −7.0 38.0 expansion coefficient(×10⁻⁷) transmission 89.5 89.5 91.9 90.0 N.A. 91.0 N.A. 90.0 (%) (400nm) (400 nm) (400 nm) (450 nm) (450 nm) (450 nm) composition SiO₂ 49 4957 60 59 60 Al₂O₃ 10 10 16 15 15 15 B₂O₃ 15 15 11 6 7 6 R₂O 0.1 2 1 2AN1(AGC) AN2(AGC) NA35(HOYA) NA45(HOYA) strain 625 616 650 610 point (°C.) thermal 44.0 47.0 39.0 48.0 expansion coefficient (×10⁻⁷)transmission 90.0 89.8 N.A. N.A. (%) (500 nm) (500 nm) composition SiO₂56 53 51 Al₂O₃ 15 11 11 B₂O₃ 2 12 13 R₂O 0.1 0.1 0.1

Where an amorphous silicon film formed on a glass substrate by CVD iscrystallized by heating, a high temperature, e.g., above 600° C., isgenerally needed. Therefore, where a Corning 7059 glass substrate isused, the substrate is shrunk by the heating.

An active-matrix liquid crystal display is known as a device utilizingTFTs formed on a glass substrate. To fabricate this liquid crystaldisplay, it is necessary to form tens of thousands to several millionsof TFTs on the glass substrate in rows and columns. To manufacture theTFTs, processes using numerous masks are necessitated. Consequently,shrinkage of the substrate is a great impediment to the manufacturingprocess.

Especially, where it is necessary to make a mask alignment beforethermal treatment, substrate shrinkage caused by the thermal treatmentis a problem.

In a process for heat-treating substrates, it is common practice toplace these plural substrates in vertical posture within a heatingfurnace, taking account of the processing speed. Where the substratesare heated above their strain point, warpage of the substrates isconspicuous.

Where TFTs are formed on a glass substrate, if the used TFTs permit flowof a large electrical current, then generation of heat accompanying theoperation is a problem. This problem associated with the heat generationarises from the difference in coefficient of thermal expansion betweensilicon and the glass substrate. That is, the coefficient of thermalexpansion of silicon, i.e., a single crystal of silicon, is 148 W m⁻¹K⁻¹(300 K), while the coefficient of thermal expansion of the glasssubstrate, i.e., quartz glass, is 1.38 W m⁻¹K⁻¹ (300 K). Since thecoefficient of thermal expansion of the glass substrate is much lowerthan that of silicon in this way, during operation of TFTs, heatgenerated by the TFTs cannot escape. Hence, malfunction and thermaldestruction due to the heat generation present problems.

Especially, these problems become conspicuous where crystalline siliconis used. In particular, TFTs using an amorphous silicon film treat weakelectrical current and so the problem of heat generation is not soserious. On the other hand, TFTs using a crystalline silicon film permitflow of large electrical current. Therefore, generation of heat poses agreat problem.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of carryingout a heating step for fabrication of a semiconductor device using aglass substrate without suffering from problems of shrinkage and flexureof the glass substrate and without suffering from the problem of heatgeneration during operation of TFTs. A film comprising aluminum nitrideis formed on one or both surfaces of a glass substrate having a strainpoint of 550-690° C. to provide a glass substrate assembly.

The present invention is based on a fact we have discovered empirically.Specifically a glass substrate was heat-treated above its strain point.Then, the substrate was slowly cooled at a rate of 0.01-0.5° C./min. Alater heating step was carried out at a temperature lower than thestrain point of the glass substrate. During this step, the substrate wasrapidly cooled at a rate of 10° C./min to 300° C./sec. Shrinkage of theglass substrate could be suppressed within 50 ppm.

By performing the above-described processing, glass substrates whichgenerally have strain points of 550-690° C. and which result inshrinkage of less than 50 ppm in a heat-treating step conducted below600° C. can be obtained.

In one feature of the invention, glass substrates are heat-treated whileheld substantially horizontal to solve the problem of warpage of theglass substrates when they are thermally treated. One example of anapparatus for holding the substrates substantially horizontal andthermally processing them is shown in FIG. 1.

FIG. 1 schematically shows a heating furnace comprising a reaction tube11 made of quartz, substrate-holding means 12, or substrate holders, andsubstrates 13 held horizontal. The apparatus is further equipped with aheater (not shown) for heating the reaction tube 11 from outside. Inaddition, the furnace is equipped with a means for supplying desiredgases into the reaction tube and also with means for moving thesubstrate-holding means out of the reaction tube.

FIG. 1 shows the condition in which the glass substrates 13 are heldhorizontal on the substrate-holding portions 12 to prevent thesubstrates from being warped and deteriorated in planarity if they areheat-treated. This scheme is advantageous where glass substrates arerequired to be heated above their strain point.

Where it is not desired to permit glass substrates to shrink in aheating step, it is possible to previously heat-treat the glasssubstrates. As a result, in a later heating step, shrinkage of thesubstrates is suppressed.

We conducted experiments and have found the following facts. If such apreheating step is carried out above the strain point of the glasssubstrates, and if they are slowly cooled after the thermal treatment,then they shrink to a large extent. If the subsequent heating step iscarried out below the transition point of the glass substrates, and ifthey are quickly cooled after this thermal processing, then they hardlyshrink. For example, a glass substrate assembly is prepared whichcomprises:

a glass substrate which shows a shrinkage of less than 50 ppm whenrapidly cooled after a heat treatment conducted at a temperature lowerthan 600° C.; and

a film provided on one or both surfaces of said glass substrate andcomprising aluminum nitride. Then, this glass substrate assembly issubjected to the heat treatment conducted at a temperature lower than600° C. This glass substrate assembly hardly shrink even by this heattreatment.

In the preheating step, it is important that each glass substrate beslowly cooled at a rate of 0.01 to 0.5° C./min, e.g., less than 0.2°C./min. A glass substrate shrinks when heated. If it is slowly cooledafter completion of heating, the substrate shrinks to a very largeextent. At the same time, local stress inside the glass substrate islessened. As a result, as the extent of shrinkage increases, shrinkageof the substrate caused by a later heating step is reduced. Furthermore,as the temperature of this heating processing is elevated, the effect ofthis method becomes more conspicuous.

In heat treatments necessary for formation of films, crystal growth, andoxidation which are carried out after the preheating processing, it isimportant that the substrate be quickly cooled at a rate of 10° C./minto 300° C./sec after the heating. Especially, around the strain point ofthe glass material, i.e., ±100° C. from the strain point, preferably±50° C. from the strain point, if the substrate is quickly cooled at theabove-described rate, then the elongation and shrinkage of the glassmaterial can be suppressed. For example, for Corning 7059 glass, in acase of a process where a processing temperature of 493-693° C. isrequired, further shrinkage (or in some cases, elongation) can beeffectively suppressed below 50 ppm by quickly cooling the substrate atleast down to 493° C.

Examples of heat treatment conducted after preheating processing includecrystallization of an amorphous semiconductor formed on the glasssubstrate, using heating, thermal annealing of semiconductor films andsemiconductor devices formed on the glass substrate, and heat treatmentneeded when semiconductor films and insulating films are formed on theglass substrate. In these heat treatment steps, heat is applied to theglass substrate.

The above-described preheating processing for causing the glasssubstrate to shrink in advance is required to be effected at atemperature higher than the heating temperature of a heating stepcarried out later. More specifically, glass substrates having strainpoints of 550-690° C. are required to be heat-treated at temperatureshigher than 600° C. and also higher than the strain points of the glasssubstrates.

An aluminum nitride (AlN) film is previously formed as a bottom film ona glass substrate to solve the problem of generation of heat from anelement (e.g. TFT) through which current flows where the element (e.g.TFT) is formed on the glass substrate.

The coefficient of thermal expansion of aluminum nitride is about 200 Wm⁻¹K⁻¹ or more and is greater than that of crystalline silicon formingTFTs. It is considered that the coefficient of thermal expansion ofcrystalline silicon is smaller than 150 W m⁻¹K⁻¹ of a single crystal ofsilicon. Hence, aluminum nitride acts as a heat dissipator, or a heatsink. In addition, an aluminum nitride film transmits visible light.Therefore, an aluminum nitride film can be used in a liquid crystaldisplay without difficulty.

Sputtering, plasma-assisted CVD, and other methods are known as methodsof forming this aluminum nitride (AlN). Especially, use of a positivecolumn type plasma-assisted CVD system capable of depositing films onboth surfaces of a glass substrate simultaneously is useful. Thermal CVDmay be used to form the aluminum nitride. As the thickness is increased,the performance of the heat sink for TFTs is improved. In practice,however, the thickness should be set to 500 Å to 3 μm, preferably 1000 Åto 1 μm.

This positive column type plasma-assisted CVD system is schematicallyshown in FIGS. 5 and 6. FIG. 6 is a cross section taken on line A-A′ ofFIG. 5. In FIGS. 5 and 6, indicated by 58 is a vacuum chamber. RF powersupplies 50 and 51 of 13.56 MHz apply RF power across a pair ofelectrodes 54 and 55 to produce an RF discharge between the electrodes.Substrates 53 on which films should be formed are held in a substratesupport 56 having a frame structure. Ends of the substrates 53 are heldinside the substrate support 56 by a substrate support member 57 suchthat films are deposited on both surfaces of each substrate. Anintroduction system 52 introduces reactive gases and a carrier gas.Unwanted gases are discharged from a discharge system 59 by a vacuumpump 60.

One advantage of the use of the system shown in FIGS. 5 and 6 is thatplural substrates can be processed at the same time. Another advantageis that films can be deposited on both surfaces of each substrate.Especially, where aluminum nitride is deposited as a bottom film on thesurface of a glass substrate, if the aluminum nitride film is formed onboth surfaces of the substrate, then the following advantages areobtained:

(1) The aluminum nitride films formed on both surfaces of the glasssubstrate can suppress the effects of substances released from the glasssubstrate.

(2) In a later heat treatment step, the substrate can be prevented fromwarping to one side.

Where the above-described advantages should be derived by ordinarysputtering techniques or by the use of parallel-plate plasma CVDequipment, a film must be formed on each side separately. When thesubstrate is turned upside down, contamination will take place. Also,the number of manufacturing steps is increased. As a result, theproductivity deteriorates.

Sometimes, an aluminum nitride film formed by sputtering techniques or aCVD process contains only a small percentage of aluminum component andan excessive amount of nitrogenous component. In this case, the grownaluminum nitride film is colored yellow. Also, the resistivity of thisfilm is not sufficient where this film is regarded as an insulator.

To improve this situation, the film is heat-treated in an ambient ofnitrogen or a mixture of nitrogen and oxygen (e.g., air). Where the filmis heat-treated in a nitrogen ambient, an aluminum nitride film havingrequired transmissivity and resistivity, of course, can be obtained.Where the film is heat-treated in an ambient consisting of a mixture ofnitrogen and oxygen, it is possible to obtain a film of aluminum nitridegiven by AlN_(x)O_(y). In this case, the film can contain 0.001 to 10atomic percent of oxygen (O). Good transmissivity and insulatingcharacteristics can be derived. Furthermore, the film can serve as aheat sink, or a cool sink, for TFTs. If local heat-generating regionssuch as TFTs exist on the substrate, the cool sink distributes thegenerated heat over the whole substrate and makes the temperatureuniform. The aluminum nitride film may also contain 0.01 to 20 atomic %of oxygen.

A step in which this aluminum nitride film is heat-treated and, at thesame time, the glass substrate is previously shrunk is quiteadvantageous to the manufacturing process. In particular, preheating ofthe aluminum nitride film and preheating of the glass substrate areeffected at the same time. As a result, an aluminum nitride film bestsuited for high-mobility TFTs is formed as a bottom film. In addition,shrinkage of the glass substrate can be suppressed to a minimum even ina step where heating is needed.

Of course, an aluminum nitride film may be formed after preheating theglass substrate, and then the aluminum nitride film may be heat-treated.In this case, it is important that the preheating be carried out abovethe strain point of the glass substrate and that the heating of thealuminum nitride film be effected below the temperature of thepreheating. At this time, the glass substrate is slowly cooled to shrinkit after the preheating. After the heating of the aluminum nitride film,the substrate is quickly cooled. Thus, shrinkage of the substrate causedduring heat-treating step of this aluminum nitride film can besuppressed to a minimum.

Our research has revealed that an amorphous silicon film can becrystallized even at a temperature lower than 600° C. by introducing animpurity such as Ni, Pb, or Si for promoting the crystallization intothe amorphous silicon film, and that crystals can be grown eitherparallel to the substrate or selectively by selectively introducing animpurity such as Ni, Pb, or Si that promotes the crystallization.

Where this manufacturing step is adopted, it is necessary to make a maskalignment before crystallization so as to selectively introduce animpurity before the crystallization utilizing heating is effected.Therefore, in this case, suppressing shrinkage of the glass substrate ina heating step below 50 ppm is quite useful for the present invention,the heating step being carried out after a mask alignment step.Depending on the kind of the glass, elongation and shrinkage exhibitanisotropy.

When an oxide film is formed on the surface of a semiconductor by meansof heating in an oxidizing ambient (this is generally known as thermaloxidation), the invention can be used to advantage. Also, where adesired film is formed by means of heating in an ambient containing theraw materials of the film, the invention can be used to advantage.

It is important that any of these heating steps be carried out at atemperature lower than the temperature of preheating processinginitially carried out above the strain point of the glass substrate. Inaddition, it is important that where preheating processing is performed,slow cooling be done after the preheating processing, and that in laterheating processing, quick cooling be effected.

Further, a method of heat-treating a glass substrate having a strainpoint in accordance with another aspect of the present inventioncomprises the steps of:

forming a film comprising aluminum nitride on a surface of said glasssubstrate; and

heat-treating said glass substrate at a temperature lying within ±50° C.from the strain point of said glass substrate so that aluminum remainingin said aluminum nitride may be nitrided or oxidized. After said step ofheat-treating the glass substrate, a second heat treatment may becarried out at a temperature lower than the temperature of said step ofheat-treating said glass substrate, followed by rapidly cooling thesubstrate at a rate of 10° C./min to 300° C./sec around the strain pointof the glass substrate.

Other objects and features of the invention will appear in the course ofthe description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross section of a heating furnace;

FIGS. 2(A)-2(D) are cross-sectional views of TFTs of Example 1 of theinvention, illustrating successive steps for fabricating the TFTs;

FIGS. 3(A)-3(C) are cross sections illustrating Example 2 of theinvention, illustrating successive steps for fabricating the TFTs;

FIGS. 4(A)-4(D) are cross sections illustrating Example 3 of theinvention, illustrating successive steps for fabricating the TFTs;

FIG. 5 is a schematic view of a CVD system used in the illustratedexamples of the invention; and

FIG. 6 is a schematic view of the CVD system shown in FIG. 5 taken online A-A′ of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION EXAMPLE 1

The present example is an example of a complementary combination of aP-channel TFT (PTFT) and an N-channel TFT (NTFT). These TFTs use acrystalline silicon film formed on a glass substrate shown in FIGS.2(A)-2(D). The configuration of the present example can be used forswitching devices for pixel electrodes for an active-matrix liquidcrystal display, a peripheral driver circuit, an image sensor, and athree-dimensional IC. Especially, where a peripheral circuit for anactive-matrix liquid crystal display is formed on the same substrate,the configuration of the present example can be advantageously used forTFTs of the peripheral driver circuit, for the following reason. TheTFTs of the peripheral driver circuit are driven with a large current.Consequently, it is necessary that high reliability be secured in spiteof heat generated.

In the present example, NA35 (see Table 1) is used as a substrate. ThisNA35 substrate has a strain point of 650° C. and is more heatresistantthan Corning 7059 glass (having a strain point of 593° C.). Hence, thisNA35 glass is useful where a high-temperature processing step is needed.

First, an aluminum nitride film 202 (AlN_(x)O_(y); y contains 0) wasformed on the glass substrate 201 to a thickness of 4000 Å by a positivecolumn type plasma-assisted CVD system shown in FIG. 5. The thickness is0.1 to 2 μm, typically 0.2 to 0.5 μm. In the present example, thethickness was 0.3 μm. For the formation of the aluminum nitride film202, Al(C₄H₉)₃ and N₂ gases were used as raw material gases. In thisstep, the aluminum nitride film 202 was formed on both surfaces of theglass substrate 201 at the same time.

Then, the glass substrate 201 was previously heat-treated to performpreheating processing for previously shrinking the glass substrate 201and simultaneously to carry out an annealing step for annealing thepreviously formed aluminum nitride film. This step was effected for 4hours at 690° C. which was higher than the strain point (650° C.) of theNA35 glass substrate 201. The ambient consisted of 100% nitrogen (N₂).The ambient may consist of ammonia (NH₃), dinitrogen monoxide (N₂O),nitrogen (N₂) or a mixture thereof. The pressure was ordinary pressure.

This step was carried out, using the heating furnace shown in FIG. 1while holding plural substrates 13 horizontal. The heating step wasperformed in a nitrogen ambient at ordinary pressure. Preferably, thisheat treatment is made within ±30 degrees from the horizontal in orderto prevent the substrates from warping.

Where aluminum nitride is deposited on both surfaces of the glasssubstrate as described above, the possibility that the substrates arewarped by heat treatment is reduced and so this heat treatment can bemade while placing the substrates upright.

After the end of the heat treatment, each glass substrate was cooled ata rate of 0.01 to 0.5° C./min, e.g., 0.2° C./min. The cooling rate wascontrolled by using a nitrogenous gas such as nitrogen gas (N₂), ammonia(NH₃), or dinitrogen monoxide (N₂O) and varying the flow rate of theintroduced gas. The nitrogen gas may contain 1 to 25 atomic % of oxygen.As a result of this slow cooling step, the glass substrate was shrunk bymore than 1000 ppm. During cooling subsequent to the preheatingprocessing, if nitrogen, ammonia, or dinitrogen monoxide is used, thenvicinities of the surface of the glass substrate can be further nitridedby the gas. Thus, when the aluminum nitride films are annealed,impurities in the glass such as boron, barium, and sodium are preventedfrom being deposited in a semiconductor formed in later steps. This iseffective in fabricating reliable semiconductor devices.

Before or after the above-described step, silicon oxide may be depositedas a bottom film on the aluminum nitride film 202. In this caseshowever, the performance of the heat sink formed by the aluminum nitridefilms deteriorates.

Also, after preheating processing for shrinking the substrate isperformed, aluminum nitride films may be formed and then thermallyannealed, e.g. by heat-treating the substrate in an ambient containingnitrogen.

Thereafter, an intrinsic (I-type) amorphous silicon film 203 having athickness of 300 to 1500 Å, e.g., 800 Å, was formed by plasma CVD. Asilicon oxide film 204 having a thickness of 100 to 800 Å, e.g., 200 Å,was deposited on the amorphous silicon film 203 by plasma CVD. This film204 would act as a protective film in a thermal annealing step conductedlater and prevent the film surface from roughening (FIG. 2(A)).

The laminate was thermally annealed at 600° C. for 8 hours in a nitrogenambient at atmospheric pressure to crystallize the amorphous siliconfilm 203. In this way, a crystalline silicon film was obtained. Then,the laminate was quickly cooled at a rate of 10 to 300° C./min, e.g.,more than 50° C./min, from the strain point by 100° C. (in this case,down to 493° C.). At this time, shrinkage of 0 to 44 ppm (on average,less than 20 ppm) of the glass substrate was observed. The heatingfurnace shown in FIG. 1 was used also for this step.

Meanwhile, some glass substrates did not undergo the preheatingprocessing conducted at 640° C. A bottom film and an amorphous siliconfilm were formed on each substrate. The laminate was thermally annealedat 600° C. for 8 hours as described above. In this case, shrinkage ofmore than 1000 ppm was observed.

Prior to the crystallization of the amorphous silicon film utilizingheating, Ni or Pb is introduced as a crystallization-promoting materialinto the top or bottom surface of the amorphous silicon film, or thematerial is injected into the amorphous silicon film by ionimplantation. In this way, crystals can be grown parallel to thesubstrate from the region in which the material has been introduced.Also, where silicon ions are selectively implanted, selective crystalgrowth can be done.

In this case, a mask must be formed and steps for creating films or anion implantation step must be carried out before the heating step whichis needed for crystallization. During the heating step, shrinkage of thesubstrate must be suppressed to a minimum. Therefore, in this case, thepresent invention capable of suppressing shrinkage of the glasssubstrate can be used to advantage.

Data about shrinkage of glass substrates each consisting of Corning 7059glass in the present example is listed in Table 2. The substrates weresubjected to preheating processing under the same conditions. Then, abottom film was formed on each substrate. Subsequently, an amorphoussilicon film was formed. The laminates were heated and crystallizedunder different conditions. Table 2 shows data about the final shrinkageof each substrate.

TABLE 2 Shrinkage of glass substrates under various conditionspreheating processing of thermal anneal shrinkage of substrate (640° C.,4 hours) (600° C., 4 hours) substrate (ppm) slow cooling rapid coolingfrom 0 (not (0.2° C./min) (50° C./min) detectable) to 44 slow coolingnormal cooling 60-90 (0.2° C./min) (1-2° C./min) slow cooling slowcooling 300 (0.2° C./min) (0.2° C./min)

As can be seen from Table 2, where a glass substrate is heated to atemperature lower than the transition point (in this case, 628° C.) ofthe glass substrate, i.e., where the heating processing temperature iswithin at least ±100° C. from the strain point of the glass, as thecooling rate is increased, the resulting shrinkage of the substrate isreduced.

After the crystallization of the amorphous silicon film 203, making useof heating, the protective film 204 was removed. The silicon film 203was photolithographically patterned to form an active layer 205 of TFTsin the form of islands. The dimension of the active layer 205 isdetermined, taking account of the channel length and the channel widthof TFTs. Smaller active layers measured 50 μm by 20 μm. Larger activelayers measured 100 μm by 1000 μm.

Then, the laminate was irradiated with infrared light having a peakbetween 0.6 to 4 μm (between 0.8 and 1.4 μm in this example) for 30 to180 seconds to anneal the active layer 205, for further enhancing thecrystallinity of the active layer 205.

At this time, the irradiation of the infrared light heated the activelayer 205 to between 800 and 1300° C., typically between 900 and 1200°C., e.g., 1100° C. This temperature was not the actual temperature onthe glass (because the glass transmits infrared light) but thetemperature on a silicon wafer used as a monitor. The infrared lightirradiation was effected in a H₂ ambient to improve the state of thesurface of the active layer. In the present example, the active layerwas selectively heated and, therefore, elevation of temperature of theglass substrate can be suppressed to a minimum. This was quite effectivein reducing the defects and dangling bonds in the active layer (FIG.2(B)).

A halogen lamp was used as an infrared light source. The intensity ofthe visible light or near-infrared light was so adjusted that thetemperature on the single-crystal silicon monitor wafer was between 800and 1300° C., typically between 900 and 1200° C. More specifically, thetemperature of a thermocouple buried in the silicon wafer was monitoredThe resulting signal was fed back to the infrared light source. Weestimate that the temperature on the surface of the silicon on the glasssubstrate has decreased to about ⅔ of the original temperature. In thepresent example, the temperature was elevated at a constant rate of 50to 200° C./sec. The temperature was rapidly lowered by natural coolingat a rate of 20 to 100° C./sec.

During irradiation of the infrared light, a silicon oxide film or asilicon nitride film is preferably formed as a protective film on thesurface. This improves the state of the surface of the silicon film 205.In the present example, in order to improve the state of the surface ofthe silicon film 205, the infrared light irradiation was effected in aH₂ ambient, which may contain 0.1 to 10 volumetric % of HCl, otherhalogenated hydrogen, a fluorine compound, a chlorine compound, or abromine compound.

This irradiation of visible light or near-infrared light selectivelyheats the crystallized silicon film and so the temperature elevation ofthe glass substrate can be suppressed to a minimum. This is quiteeffective in reducing the defects and dangling bonds in the siliconlayer. After the end of this step, defects can be effectively reduced bycarrying out a hydrogen anneal at 200 to 500° C., typically 350° C. Thesame advantage can be obtained by implanting hydrogen ions at a dose of1×10¹³ to 1×10¹⁵ cm⁻² and performing thermal processing at 200 to 300°C.

After the irradiation of the infrared light described above, a siliconoxide film 206 having a thickness of 1000 Å was formed as agate-insulating film by plasma CVD. TEOS (tetraethoxy-silane(Si(OC₂H₅)₄)) and oxygen were used as raw material gases in the plasmaCVD. During the growth of the film, the substrate temperature was 300 to550° C., e.g., 400° C.

After the formation of the silicon oxide film 206 becoming agate-insulating film, an optical anneal using irradiation of visiblelight or near-infrared light was carried out under the same conditionsas in the aforementioned step of irradiating infrared light. This annealcould annihilate mainly energy levels at and around the interfacebetween the silicon oxide film 206 and the silicon film 205. This isquite useful for an insulated gate field-effect semiconductor device forwhich the characteristics of the interface between the gate-insulatingfilm and the channel formation region are quite important.

Subsequently, an aluminum film having a thickness of 6000 to 8000 Å,e.g., 6000 Å, was formed by sputtering techniques. The aluminumcontained 0.01 to 0.25 % by weight of a rare-earth element belonging togroup IIIa of the Periodic Table. Instead of aluminum, an elementbelonging to group IIIb can be used. The aluminum film was patterned toform gate electrodes 207 and 209. The surfaces of the aluminumelectrodes were anodized to form oxide layers 208 and 210 on thesurfaces. This anodic oxidation was effected within an ethylene glycolsolution containing 1 to 5% tartaric acid. The thickness of the obtainedoxide layers 208 and 210 was 2000 Å. These oxide layers 208 and 210determine the thickness of an offset gate region in a later ion dopingstep and so the length of the offset gate region can be determined inthe anodic oxidation step.

Then, using a gate electrode portion (comprising the gate electrode 207,the surrounding oxide layer 208, the gate electrode 209, and thesurrounding oxide layer 210) as a mask, an impurity was implanted intothe silicon film 205 to impart conductivity type P or N by self-aligningtechniques and by ion doping (also known as plasma doping). Phosphine(PH₃) and diborane (B₂H₆) were used as dopant gases. Where the phosphinewas used, the accelerating voltage was 60 to 90 kV, e.g., 80 kV. Wherediborane was used, the accelerating voltage was 40 to 80 kV, e.g., 65kV. The dose was 1×10¹⁴ to 8×10¹⁵/cm². For example, phosphorus wasimplanted at a dose of 2×10¹⁵/cm². Boron was implanted at a dose of5×10¹⁵/cm². During the implantation, one region was coated with aphotoresist to selectively implant the elements. As a result, N-typedoped region 216, 216 and P-type doped regions 211, 213 were formedincluding respective channel regions 212 and 215. In this way, a regionfor a P-channel TFT (PTFT) and a region for an N-channel TFT (NTFT)could be formed.

Thereafter, the laminate was annealed by irradiation of laser light. Thelaser light was emitted by a KrF excimer laser having a wavelength of248 nm and a pulse width of 20 nsec. Other lasers can also be used. Thelaser light was emitted at an energy density of 200 to 400 mJ/cm², e.g.,250 mJ/cm². Two to 10 shots of laser light were emitted per location.For example, two shots were emitted. During illumination of this laserlight, if the substrate is heated to about 200 to 450° C., then theeffect can be enhanced (FIG. 2(C)).

This step may utilize lamp annealing that makes use of visible light ornear-infrared light. Visible and near-infrared radiation is easilyabsorbed by crystallized silicon and by amorphous silicon doped withphosphorus or boron at a dose of 10¹⁷ to 10²¹ cm⁻³. This lamp anneal iscomparable to thermal annealing carried out above 1000° C. If phosphorusor boron is added, the impurity scatters light. Therefore, even infraredlight is sufficiently absorbed. This can be sufficiently estimated fromthe fact that the silicon is observed to be black with the naked eye. Onthe other hand, near-infrared light is not readily absorbed by the glasssubstrate and so the substrate is not overheated. Furthermore, theprocessing can be performed in a short time. Therefore, it can be saidthat this is the best method for a step where shrinkage of glasssubstrates is a problem.

Subsequently, a silicon oxide film 217 having a thickness of 6000 Å wasformed as an interlayer insulator by plasma CVD. This interlayerinsulator may also be made of either polyimide or a two-layer filmconsisting of silicon oxide and polyimide. Then, contact holes wereformed. Electrodes and interconnects 218, 220, and 219 of TFTs werefabricated out of a metal material such as a multilayer film of titaniumnitride and aluminum. Finally, the laminate was annealed at 350° C. for30 minutes in a hydrogen ambient at 1 atm to complete semiconductorcircuitry having complementary TFTs (FIG. 2(D)).

The circuitry described above is of the CMOS structure comprisingcomplementary PTFT and NTFT. In the above step, it is also possible tofabricate the two TFTs simultaneously. Then, the substrate is cut at thecenter to obtain two independent TFTs at the same time.

EXAMPLE 2

The present example is an active-matrix liquid crystal displaycomprising N-channel TFTs which are formed at pixels as switchingdevices. In the description made below, only one pixel is described butmany other (generally hundreds of thousands of) pixels are formedsimilarly. Obviously, the TFTs are not limited to N-channel TFTs.P-channel TFTs can also be used.

The process sequence of the present example is schematically shown inFIGS. 3(A)-3(C). In the present example, Corning 7059 glass was used asa substrate 400. This substrate had a thickness of 1.1 mm and measured300 mm by 400 mm. First, an aluminum nitride film 401 was formed to athickness of 1000 to 5000 Å, typically 2000 Å, by sputtering or reactivesputtering techniques. Since the aluminum nitride film has a high degreeof transparency and suppresses movement of ions, the film effectivelyprevents mobile ions from diffusing from the substrate 400 into the TFTregions. A silicon oxide film 402 was formed as a bottom film to athickness of 2000 Å by plasma CVD.

Then, the laminate was heat-treated at 640° C. for 4 hours within anitrogen ambient. Subsequently, the laminate was slowly cooled at a rateof 0.1° C./min within ammonia. This step produces desirable results ifthe rate is less than 0.5° C./min. In this step, the glass substratehaving the bottom film thereon can be shrunk previously. Also duringthis step, the heating furnace shown in FIG. 1 was used.

Thereafter, an amorphous silicon film having a thickness of 1000 Å wasformed by plasma CVD. A mask was then fabricated out of a photoresist.Silicon ions were implanted to a portion becoming a channel formationregion. At this time, the projected range of implanted silicon ions werelocated in the vicinities of the center of the silicon film.

Subsequently, the mask was removed, and the laminate was thermallyannealed at 550° C. for 8 hours. During this step, only the regionsalready doped with the silicon ions were crystallized. After this step,the laminate was rapidly cooled at a rate exceeding 50° C./min. Thisstep may also utilize a method of consisting of taking the laminate outof the heating furnace and cooling the laminate by natural cooling.

Then, the silicon film was photolithographically patterned to leavebehind the active layer 403 of a TFT in the form of an island. Thecentral portion of this island 403 was already implanted with siliconions and would become a channel formation region. In this structure,only the channel formation region had a high degree of crystallinity.

The active layer 403 in the form of an island was irradiated withvisible light or near-infrared light in an ambient of oxygen ordinitrogen monoxide to improve the crystallinity of the silicon film. Asilicon oxide film 404 having a thickness of 50 to 200 Å, typically 100Å, was formed on the surface of the island. The temperature was 1100°C., and the time was 30 seconds. This step of forming the silicon oxidefilm 404 may also be carried out by heating the laminate to 550 to 650°C. in an ambient of oxygen or dinitrogen monoxide. Obviously, theapparatus shown in FIG. 1 was used for this purpose (FIG. 3(A)).

Then, a gate-insulating film 406 having a thickness of 500 to 3000 Å,typically 1200 Å, was formed out of aluminum nitride either by asputtering, method using a target of aluminum nitride or by a reactivesputtering method using a target of aluminum. The substrate temperaturewas 350° C. As a result, two layers were created. That is, one layer wasthe thin silicon oxide film 404 formed by thermal oxidation, while theother was the aluminum nitride film 406 formed by sputtering. Since thedielectric constant of aluminum nitride is more than five times as greatas the dielectric constant of silicon oxide, the use of aluminum nitrideis effective in reducing the threshold voltages of TFTs, especially thethreshold voltage of P-channel TFTs. Furthermore, less localized centersare produced in aluminum nitride than in silicon nitride and so aluminumnitride is preferable as the material of a gate-insulating film.Thereafter, the aluminum nitride film 406 was annealed by thermalannealing within a nitrogen ambient or by lamp annealing.

Thereafter, a film consisting mainly of well-known polysilicon wasformed by LPCVD and photolithographically patterned to form a gateelectrode 407. In order to improve the conductivity, 0.1 to 5 atomic %of phosphorus was added as a dopant to the polysilicon (FIG. 3(B)).

Then, phosphorus was implanted as an N-type dopant by ion doping, and asource region 408, a channel formation region 409, and a drain region410 were formed simultaneously by self-aligning techniques. The laminatewas irradiated with light emitted by a KrF laser to improve thecrystallinity of the silicon film which was deteriorated by the ionimplantation. The energy density of the laser light was 250 to 300mJ/cm². As a result of this laser irradiation, the sheet resistance ofthe source/drain regions of the TFTs assumed a value of 300 to 800Ω/cm². In the case of a lightly doped drain structure (LDD) having adose lower than normal source/drain regions, the sheet resistance was 10to 200 kΩ/•. The laser annealing step may be replaced by lamp annealingmaking use of visible light or near-infrared light.

Thereafter, an interlayer insulator 411 was formed out of silicon oxideor polyimide. Then, a pixel electrode 412 was formed out of ITO. Contactholes were formed. Electrodes 413 and 414 were formed out of amultilayer film of chromium and aluminum in the source/drain regions ofa TFT. One electrode 414 was connected also with the ITO pixel electrode412. Finally, the laminate was annealed at 200-400° C. for 2 hourswithin hydrogen to carry out hydrogenation. In this way, one TFT wascompleted (FIG. 3(C)).

EXAMPLE 3

The present example is described by referring to FIGS. 4(A)-4(D).Corning 7059 glass was used as a substrate 501. An aluminum nitride film(not shown) having a thickness of 5000 Å was formed on each side of thesubstrate by the positive column type plasma-assisted CVD system shownin FIGS. 5 and 6. To anneal the aluminum nitride film and to preventshrinkage, the glass substrate was previously annealed at 640° C. for 4hours within nitrogen. Then, the substrate was slowly cooled down to450° C. at a rate of 0.1° C./min within nitrogen. Thereafter, thesubstrate was taken out of the heating furnace.

As a result of this step, the glass substrate shrank to a large extent.Shrinkage caused by a later heating step could be suppressed within 50ppm. At the same time, the aluminum nitride (AlN) film could beannealed. Its insulation and transmissivity could be improved.

First, a bottom film 502 was formed on the substrate 501. An amorphoussilicon film having a thickness of 300 to 800 Å was formed by plasmaCVD. The laminate was thermally annealed at 600° C. for 1 hour.Thereafter, the laminate was rapidly cooled down to 450° C. at a rate of2 to 200° C./sec, preferably at a rate in excess of 10° C./sec toprevent the substrate from shrinking. If the used heating furnace doesnot permit this rapid cooling, then the same advantage can be obtainedby taking the substrate out of the furnace and maintaining the substrateat room temperature. Also during this step, the heating furnace shown inFIG. 1 was used.

In the present example, since the thermal annealing temperature washigher than the strain point (593° C.) of the Corning 7059 glass, it wasdifficult to suppress shrinkage even if previous heat treatment and slowcooling were carried out. In this case, rapid cooling from the annealingtemperature described above produces desirable effects.

Then, the silicon film was photolithographically patterned to formactive layer regions 505 and 506 in the form of islands. The activelayer regions were etched by RIE having vertical anisotropy (FIG. 4(A)).

Thereafter, a silicon oxide film or silicon nitride film 507 having athickness of 200 to 3000 Å was formed by plasma CVD. Where the siliconoxide film was formed, LPCVD or photo-assisted CVD may also be used.Then, the laminate was treated with visible light or near-infrared lightunder the same conditions as in Example 1. In the present example,during irradiation of visible light or near-infrared light, a protectivefilm of silicon oxide or silicon nitride was formed on the surfaces ofthe active layers. This could prevent the surface from roughening orbecoming contaminated during irradiation of infrared light (FIG. 4(B)).

After the irradiation of the visible light or near-infrared light, theprotective film 507 was removed. Thereafter, a gate-insulating film 508,a gate electrode, a surrounding oxide layer 509, another gate electrode,and a surrounding oxide layer 510 were formed, in the same way as inExample 1. Doped regions were formed by ion doping and activated bylaser irradiation (FIG. 4(C)).

Thereafter, an interlayer insulator 511 was formed, and contact holeswere formed in the insulator. Metallic interconnects 512, 513, and 514were formed (FIG. 4(D)).

In this way, a complementary TFT circuit was completed. In the presentexample, during irradiation of visible light or near-infrared light, aprotective film was formed on the surfaces of the active layers. Thiscould prevent the surfaces from roughening or becoming contaminatedduring irradiation. Therefore, TFTs of the present example had quiteexcellent characteristics, such as excellent field mobility andthreshold voltage, and quite high reliability. Furthermore, as can beseen from the present example, the present invention can be applied withespecially great utility to glass substrate materials having strainpoints of 550 to 650° C. In the present invention, if the slow coolingstep is carried out within an ambient of a nitrogenous gas such asnitrogen, ammonia, or dinitrogen monoxide, then the glass is nitrided.This suppresses diffusion and deposition of various impurity elementscontained in the glass onto the glass surface. Hence, semiconductordevices having high reliability can be manufactured.

In accordance with the present invention, a glass substrate ispreviously heat-treated above the transition point. Then, the substrateis slowly cooled to cause it to shrink. Subsequently, the substrate isheat-treated below the strain point of the substrate. The substrate isthen rapidly cooled. Thus, shrinkage of the glass substrate occurring atthis time can be reduced to a minimum.

An aluminum nitride (AlN) film is formed as a bottom film for asemiconductor film. As a result, TFTs can have margins for heatgenerated. This can enhance the reliability and stability of a deviceusing TFTs such as an active-matrix liquid crystal display.

Furthermore, by carrying out the step for preheating the glass substratein an ambient consisting of a mixture of nitrogen and oxygen, theprocessing for previously shrinking the glass substrate can be effectedsimultaneously with the thermal annealing step for enhancing theinsulation and transmissivity of the aluminum nitride film. This isquite useful to the manufacturing process.

In the illustrated examples, the description centers on Corning 7059glass substrate. Obviously, other glass substrates such as Corning 1733,HOYA LE30, HOYA NA35, HOYA NA45, OA2 manufactured by Nippon ElectricGlass (NEG) Co., Ltd., Asahi Glass AN1, and Asahi Glass AN2 which arelisted in Table 1 can produce similar effects.

What is claimed is:
 1. A semiconductor device comprising: a substratehaving a front surface and a rear surface; an insulating film comprisingaluminum nitride containing 0.001 to 10 atomic percent of oxygenprovided on said front surface of the substrate; and a transistorprovided over said front surface of the substrate, said transistorhaving at least a channel formation region comprising crystallinesilicon, a gate insulating film adjacent to said channel formationregion, and a gate electrode adjacent to said channel formation regionwith said gate insulating film interposed therebetween.
 2. The device ofclaim 1 wherein said substrate is a glass substrate.
 3. A semiconductordevice comprising: a substrate having a front surface and a rearsurface; an insulating film comprising aluminum nitride containing 0.001to 10 atomic percent of oxygen provided on said front surface of thesubstrate; and a transistor provided over said front surface of thesubstrate, said transistor having at least a channel formation regioncomprising crystalline silicon, a gate insulating film adjacent to saidchannel formation region, and a gate electrode adjacent to said channelformation region with said gate insulating film interposed therebetween,wherein said insulating film comprising aluminum nitride has a thicknessof 500 Å to 3 μm.
 4. The device of claim 3 wherein said substrate is aglass substrate.
 5. A semiconductor device comprising: a substratehaving a front surface and a rear surface; an insulating film comprisingaluminum nitride containing 0.001 to 10 atomic percent of oxygenprovided on said front surface of the substrate; and a transistorprovided over said front surface of the substrate, said transistorhaving at least a channel formation region, a gate insulating filmadjacent to said channel formation region, and a gate electrode adjacentto said channel formation region with said gate insulating filminterposed therebetween, wherein said insulating film comprisingaluminum nitride has a thermal conductivity of 200 Wm⁻¹K⁻¹ or more. 6.The device of claim 5 wherein said AlN_(x)O_(y) layer has a thickness of500 Å to 3 μm.
 7. The device of claim 5 wherein said substrate is aglass substrate.
 8. A semiconductor device comprising: a substratehaving a front surface and a rear surface; an insulating film comprisingaluminum nitride containing 0.001 to 10 atomic percent of oxygenprovided on said rear surface of the substrate; and a transistorprovided over said front surface of the substrate, said transistorhaving at least a channel formation region comprising crystallinesilicon, a gate insulating film adjacent to said channel formationregion, and a gate electrode adjacent to said channel formation regionwith said gate insulating film interposed therebetween.
 9. The device ofclaim 8 wherein said insulating film comprising aluminum nitridecontaining 0.001 to 10 atomic percent of oxygen has a thickness of 500 Åto 3 μm.
 10. The device of claim 8 wherein said substrate is a glasssubstrate.
 11. An active matrix type liquid crystal display comprising:a substrate having a front surface and a rear surface; an insulatingfilm comprising aluminum nitride containing 0.001 to 10 atomic percentof oxygen provided on said front surface of the substrate; and atransistor provided over said front surface of the substrate, saidtransistor having at least a channel formation region comprisingcrystalline silicon, a gate insulating film adjacent to said channelformation region, and a gate electrode adjacent to said channelformation region with said gate insulating film interposed therebetween.12. The display of claim 11 wherein said insulating film comprisingAlN_(x)O_(y) has a thickness of 500 Å to 3 μm.
 13. The display of claim11 wherein said substrate is a glass substrate.
 14. An active matrixtype liquid crystal display comprising: a substrate having a frontsurface and a rear surface; an insulating film comprising aluminumnitride containing 0.001 to 10 atomic percent of oxygen provided on saidrear surface of the substrate; and a transistor provided over said frontsurface of the substrate, said transistor having at least a channelformation region comprising crystalline silicon, a gate insulating filmadjacent to said channel formation region, and a gate electrode adjacentto said channel formation region with said gate insulating filminterposed therebetween.
 15. The display of claim 14 wherein saidinsulating film comprising aluminum nitride containing 0.001 to 10atomic percent of oxygen has a thickness of 500 Å to 3 μm.
 16. Thedisplay of claim 14 wherein said substrate is a glass substrate.
 17. Anactive matrix type liquid crystal display comprising: a substrate havinga front surface and a rear surface; an insulating film comprisingaluminum nitride containing 0.001 to 10 atomic percent of oxygenprovided on said front surface of the substrate; and a transistorprovided over said front surface of the substrate, said transistorhaving at least a channel formation region comprising crystallinesilicon, a gate insulating film adjacent to said channel formationregion, and a gate electrode adjacent to said channel formation regionwith said gate insulating film interposed therebetween, wherein saidinsulating film comprising aluminum nitride has a thermal conductivityof 200 Wm⁻¹K⁻¹ or more.
 18. The display of claim 17 wherein saidinsulating film comprising aluminum, has a thickness of 500 Å to 3 μm.19. An active matrix type liquid crystal display comprising: a substratecomprising a glass having a front surface and a rear surface; aninsulating film comprising aluminum nitride containing 0.001 to 10atomic percent of oxygen provided on said front surface of thesubstrate; and a transistor provided over said front surface of thesubstrate, said transistor having at least a channel formation regioncomprising crystalline silicon, a gate insulating film adjacent to saidchannel formation region, and a gate electrode adjacent to said channelformation region with said gate insulating film interposed therebetween,wherein said insulating film comprising aluminum nitride has a thermalconductivity of 200 Wm⁻¹K⁻¹ or more.
 20. The display of claim 19 whereinsaid insulating film comprising aluminum, nitrogen and oxygen has athickness of 500 Å to 3 μm.
 21. An active matrix type liquid crystaldisplay comprising: a substrate having a front surface and a rearsurface; an insulating film comprising aluminum nitride containing 0.001to 10 atomic percent of oxygen provided on said rear surface of thesubstrate; and a transistor provided over said front surface of thesubstrate, said transistor having at least a channel formation regioncomprising crystalline silicon, a gate insulating film adjacent to saidchannel formation region, and a gate electrode adjacent to said channelformation region with said gate insulating film interposed therebetween,wherein said insulating film, comprising aluminum nitride containing0.001 to 10 atomic percent of oxygen, has a thermal conductivity of 200Wm⁻¹K⁻¹ or more.
 22. The display of claim 21 wherein said insulatingfilm comprising aluminum nitride containing 0.001 to 10 atomic percentof oxygen has a thickness of 500 Å to 3 μm.
 23. The display of claim 21wherein said substrate is a glass substrate.
 24. An active matrix typeliquid crystal display comprising: a substrate comprising a glass havinga front surface and a rear surface; an insulating film comprisingaluminum nitride containing 0.001 to 10 atomic percent of oxygenprovided on said front surface of the substrate; and a transistorprovided over said front surface of the substrate, said transistorhaving at least a channel formation region comprising crystallinesilicon, a gate insulating film adjacent to said channel formationregion, and a gate electrode adjacent to said channel formation regionwith said gate insulating film interposed therebetween, wherein saidinsulating film comprising aluminum nitride has a thickness of 500 Å to3 μm.
 25. The display of claim 24 wherein said substrate is a glasssubstrate.
 26. A semiconductor device comprising: a substrate having afront surface and a rear surface; an insulating film comprising aluminumnitride containing 0.001 to 10 atomic percent of oxygen provided on saidfront surface of the substrate; and a transistor provided over saidfront surface of the substrate, said transistor having at least achannel formation region comprising crystalline silicon, a gateinsulating film adjacent to said channel formation region, and a gateelectrode adjacent to said channel formation region with said gateinsulating film interposed therebetween, wherein said insulating filmcomprising aluminum nitride has a thickness of 500 Å to 3μm, and whereinsaid insulating film comprising aluminum nitride has a thermalconductivity of 200 Wm⁻¹K⁻¹ or more.
 27. The display of claim 26 whereinsaid substrate is a glass substrate.